The Perseid meteor shower, which occurs roughly between August 10 and August 14 every year and is most prominent in the northern hemisphere, is the result of the Earth flying through a cloud of meteors and debris left by the Swift-Tuttle comet. It draws its name from the Perseus constellation, which is where the meteors appear to come from as they enter Earth’s atmosphere, causing their hallmark flaring paths through the sky as they are burned up.
However, the meteors in question actually have nothing to do with Perseus, as such – the closest star in that constellation is still many light-years away and plays no part in the internal affairs of our own solar system. Instead, all meteor showers are caused when the Earth’s orbit brings it through an unusually dense cloud of small orbiting matter and debris. On any given night, a few such small chunks, known as meteors, will enter Earth’s atmosphere and burn up. However, because of the greater density of objects present during a meteor shower, “shooting stars” temporarily become frequent visible occurrences rather than rare events. At the height of the Perseids, dozens of meteors can be seen plunging into the Earth’s atmosphere every hour.
The Perseid meteor shower has been observed and recorded by human beings for at least two thousand years, but its cause was not pinpointed until the mid-19th century, with the discovery of the comet 109P/Swift-Tuttle by Lewis Swift and Horace Parnell. Like all comets, Swift-Tuttle’s roughly 134-year orbit brings it past the Earth to about half as close to the Sun as our own planet’s orbit, and then swings it outward to between the orbits of Uranus and Neptune. During its next pass, in 2126, it is expected to be close enough to be highly visible on its own, as several comets have been in the last few decades. It is also the largest object to regularly pass this close to Earth (several times as large as the asteroid which caused the extinction of the dinosaurs), so astronomers are attempting to project its orbit as far into the future as possible to rule out any likelihood of impact.
Comets are unstable bodies made up of a mixture of ice and rock. As they approach the Sun during the inward leg of their orbit, light from the Sun melts away their surface, causing relatively small amounts of rock to break free as well as releasing a plume of vapour which appears from Earth as a distinct smudge or halo, known as a coma. In the case of a particularly large and stable comet like Swift-Tuttle, this process has been going on for quite some time and a substantial body of material has been released. This stream of debris forms the cloud of small objects through which Earth passes in August of every year. Each time, some of the material actually collides with Earth, causing the meteor shower. Every 134 years, Swift-Tuttle passes through and replenishes the cloud.
